I really like Mutants and Masterminds as a system, honestly. It's very elegant for its purpose, and is surprisingly complete with just the core book (something not many games can say). You could use Tristat or GURPS or whatever, I'm sure, but the easy-to-understand nature of d20 is a wonderful base. Of course, I'm biased since I was introduced to role-playing via D&D 3.5e.
Now the thing that really impresses me is how variable it is for the setting's wacked out factors. Ultra powerful heroes a la DC? Just set a really high power level like 20. Want to do something more down to earth? Lower power levels like 6-8. The suggested default of 10 is really broad, too, and the archetypes they give you as examples are great for suggesting how to start abusing the system. While D&D books had those sample class character sheets, they weren't nearly as useful as archetypes (what with the fact that you will never use those as a basis for a character and everything.)
The balance is pretty great as well, from what I see. I don't think it's perfect, there's probably a few abuse spots in it, but it's not hard to make a character of any sort that's useful. Batman isn't limited because he doesn't have powers in this system, which is how it should always be. The sheer number of options available is great, as well. The only thing I can see is that you need to be a bit creative to recreate certain things; I tried to recreate Captain America and noticed there wasn't anything for catching thrown stuff after you throw it, so I had to recreate his throwing ability as a blast power in his shield (I was thinking strike but they sort of suggested using blast in the text).
Overall I'm going to have a lot of fun when I DM this; there are lots of opportunities to just throw out crazy thanks to the setting type, and it's a much easier system to fudge numbers for. My only regret is that I'm not going to be a player.
